Such particle filters are needed for cleaning the exhaust gases of internal combustion engines, especially in vehicles operated with diesel fuel, in order to filter out the soot particles, which are hazardous for health and cause a high level of hazardous environmental pollution. The soot separators, which are also often incorrectly called filters, should be distinguished from these particle filters; in these soot separators, when used in exhaust gas treatment units of motor vehicles, the soot is separated from the exhaust gas stream while it flows through a housing, e.g., by deflection and guidance through a filter-like lining, and is subsequently collected in a collection chamber. These soot separators require periodic emptying of the collection chamber and are therefore unsuitable for continuous operation, e.g., in utility vehicles, Such a soot separator is disclosed in, e.g., German Utility Model No. DE-GM 86,00,167.
Various embodiments of particle filters suitable for driving operation have been known. In one embodiment, the exhaust gas is filtered while it is passing through a ceramic block (monolith), which has a plurality of passage canals, of which one canal each is closed on the inlet side and another canal is closed on the outlet side in a checkerboard pattern, so that the exhaust gas flows into one canal, after which it flows into the adjacent canal through the surrounding monolith section acting as a filter, and leaves it, freed of soot particles. Such a soot filter is disclosed in, e.g., German Offenlegungsschrift No. DE-OS 32,17,357.
Another embodiment uses filter cartridges for the particle filtration. These filter cartridges are preferably designed as wound filters. Several layers of threads of a filter material are wound around a support tube, provided with exhaust gas passage openings, so that an element comparable to a textile thread spool is formed. Such an arrangement is disclosed in, e.g., German Offenlegungsschrift No. DE-OS 38,15,148, in which the mounting of the filter cartridges in mounting plates is described, in particular. The filter cartridges may also be formed by a tube of filter material pulled over a support tube corresponding to German Offenlegungsschrift No. DE-OS 38,23,205. The exhaust gas to be cleaned passes through the filter cartridges from the outside to the inside, and it passes through the filter material into the support tubes that are closed on the inlet side; the soot particles are retained during the passage of the exhaust gas through the wound filter material; the cleaned exhaust gas flows through the support tubes and into the discharge funnel, and is removed from there. The filter cartridges are of uniform design. It is also possible to arrange a mounting grid instead of the inlet-side mounting plates provided with exhaust gas passage openings.
Soot buildup occurs, i.e., the filter material is increasingly clogged by soot particles, during the operation of the internal combustion engine, and the soot must be removed after a relatively short operating time. Mechanical removal is ruled out for practical reasons, because it would make it necessary to demount the entire filter and remove the soot from it. It is therefore necessary to resort to burning free the accumulated soot. To do so, in a process that is employed in practice, oxidizing agents are provided as additives to the exhaust gas from a storage container, so that the soot collected comes into contact with these agents, which lower its ignition point and increase the rate of combustion, and can be burned off at a relatively low exhaust gas temperature. However, this manner of burning free the soot collected has the disadvantage that the additives are chemical compounds which are readily inflammable, and the exhaust gas will be enriched in undesired components, whose environmental safety has not yet been proven. Therefore, attempts have been made to connect a diesel burner as an external heat source, but it was impossible to regenerate the filters with the prior-art arrangement during the driving operation of the vehicle. Therefore, this solution requires the use of very large filters to obtain a storage capacity that permits a rather long driving operation between the phases of regeneration. This resting-phase regeneration is possible in vehicles which operate intermittently, e.g., buses used in inner-city transportation. In other vehicles, regeneration must be carried out during driving operation. For example, two filters are arranged in parallel to one another for this purpose, and one of the two filters is alternatingly regenerated.
If only one particle filter with a plurality of filter cartridges is used, it is only possible to connect--as soon as a thicker coating with soot particles is determined, e.g., from the exhaust gas counterpressure--an external heat source, which heats the exhaust gas to be cleaned (&gt;600.degree. C.) to the extent that the burning-free process can take place. However, it was found that the filter cartridges are not burned free over their entire length, especially in load states with limited exhaust gas flow. This is due to the fact that even during burning free, for example, the front half of the filter cartridges provides for a sufficiently low flow resistance so that the flow through the rear part or the outermost filter cartridges is insufficient. This incomplete regeneration leads to increasingly shorter soot buildup times, which may lead to failure of the filter in the worst case. Experiments have shown that the soot buildup time decreased from, e.g., 135 minutes at the time of the first soot buildup to 15 to 20 minutes.
To achieve a substantial improvement during regeneration by burning free, an improvement of regeneration by burning free has been achieved according to DE 40,04,861 A1 by arranging--in the space between the filter cartridges--so-called overflow tubes, via which the exhaust gas to be cleaned is fed in such that uniform admission over the entire length of the filter cartridge is achieved.
DE 38,36,697 A1 discloses a device of this class, in which the mounting plates and the filter cartridges form a rotatable drum, so that one segment of this drum can always be brought into a zone in which burning free takes place. Such a device is very expensive, especially in terms of the mounting of the rotatable drum and the sealing against the bypassing of non-cleaned exhaust gas. These problems are solved in this document. However, such devices in which burning free takes place segment by segment are unsuitable for mass-production installation.
Furthermore, German Patent Application No. P 40,26,275.4, which had not previously been published, discloses a particle filter of this class, in which filter cartridges of different active lengths are arranged distributed at equal distances from one another over the cross section of the housing. The "active length" is defined as the section of the filter cartridge lined with filter material. A different active length can be achieved by making the filter material lining different over the length of the filter cartridge, e.g., making it thicker at the front when viewed in the direction of flow than in the area following the front area. This lining of different thickness may be different in the individual filter cartridges. For example, it is possible to select a different lining in the edge zone of the filter than in the central (middle) area. The thickness of the lining may be selected according to the flow profile of the exhaust gas to be cleaned. This arrangement requires filter cartridges with different linings for each filter, which makes it necessary to stock a large amount of filter cartridges.